1. Field of the Invention.
The present invention relates to active pixel sensor cells and, more particularly, to a single MOS transistor active pixel sensor cell that provides automatic anti-blooming and wide dynamic range.
2. Description of the Related Art.
Conventional imaging circuits rely on charge-coupled devices (CCDs) to convert a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to collect the light energy, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.
Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based microprocessors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use active pixel sensor cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photogate is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1 shows an example of a conventional CMOS active pixel sensor cell 10. As shown in FIG. 1, cell 10 includes a photogate PG, a reset transistor RT, and a transfer gate TX formed between the source of reset transistor RT and photogate PG. In addition, active pixel sensor cell 10 also includes a sense transistor ST, whose gate is connected to the source of reset transistor RT, and an access transistor AT.
Operation of active pixel sensor cell 10 is performed in two steps; image integration, where the light energy is collected by photogate PG, and signal readout, where the collected energy is converted into an electrical signal and read out.
During image integration, a positive voltage (5 volts) is applied to the photogate PG, while a smaller positive voltage (2.5 volts) is applied to the transfer gate TX. Under these conditions, light energy, in the form of photons, strikes the substrate under photogate PG which, in turn, creates a number of electron-hole pairs. Due to the positive voltage applied to photogate PG, the photogenerated electrons are collected under the photogate PG.
At the same time, a positive voltage (2.5 volts) is applied to the gate of reset transistor RT to control anti-blooming by allowing excess charge to flow to the drain of reset transistor RT. In addition, access transistor AT is turned off.
Following image integration, active pixel sensor cell 10 is read out by first enabling access transistor AT. Next, the source of reset transistor RT is reset by briefly pulsing the gate of reset transistor RT with a positive voltage (5 volts). This resets the source of reset transistor RT to an initial voltage (approximately 3.5 volts).
Following this, photogate PG is pulsed low (0 volts) to transfer the signal charge stored under photogate PG to the source of reset transistor RT which, in turn, modulates the current flowing through sense transistor ST.
One of the principal advantages of active pixel sensor cell 10, in addition to providing amplification and reduced size, is that the fabrication process is inherently CMOS since only MOS transistors are utilized. However, although cell 10 is smaller than conventional CCDs, there is still a need to further reduce the size of active pixel sensor cells.